1. Field of the Invention
This invention is directed to an improved servo valve of the type employed, for example, for actuating a pressure booster of a fuel injector.
2. Description of the Prior Art
For supplying combustion chambers of self-igniting internal combustion engines with fuel, both pressure-controlled and stroke-controlled injection systems may be employed. As fuel injection systems, not only unit fuel injectors and pump-line-nozzle units but also reservoir injection systems are used. Advantageously, reservoir injection systems (common rails) make it possible to adapt the injection pressure to the load and rpm of the engine. To attain high specific performance and to reduce emissions from the engine, an injection pressure that is as high as possible is generally required.
For the sake of durability, the attainable pressure level in reservoir injection systems in current use is presently limited to about 1600 bar. To further increase the pressure in reservoir injection systems, pressure boosters are employed with them.
German Patent Disclosure DE 101 23 910.6 refers to a fuel injection system with which fuel is delivered to the combustion chambers of a multi-cylinder internal combustion engine. Each of the combustion chambers of the engine are supplied with fuel via respective fuel injectors. The fuel injectors are subjected to a high-pressure source; the fuel injection system of DE 101 23 910.6 moreover includes a pressure booster, which has a movable pressure booster piston that divides a chamber which can be connected to the high-pressure source from a high-pressure chamber that communicates with the fuel injector. The fuel pressure in the high-pressure chamber can be varied by filling a differential pressure chamber of the pressure booster with fuel or emptying this differential pressure chamber of fuel. Triggering the pressure booster via its differential pressure chamber makes it possible to keep the triggering losses in the high-pressure fuel system less in comparison with triggering via a work chamber communicating intermittently with the high-pressure source. Moreover, the high-pressure chamber of the pressure booster can be relieved only down to the pressure level of the high-pressure reservoir, rather than down to the leakage pressure level. Thus on the one hand the hydraulic efficiency can be improved, and on the other a faster pressure buildup to the system pressure level can be accomplished, so that the time intervals between individual injection phases can be shortened considerably.
A pressure booster can be used on each fuel injector in an internal combustion engine, to increase the injection pressure. If the pressure booster is not activated, a fluidic communication exists from the pressure reservoir to the injection nozzle. Such a system may be equipped with two valves with independently activatable actuators, to assure flexible shaping of the injection course. A disadvantage of this version is the relatively high production cost for controlling such a fuel injection system, with two valves and two independently activatable actuators. Because of the high diverted quantities from the differential pressure chamber of the pressure booster, embodying a pressure booster control valve necessitates the use of a servo-hydraulically supported valve. However, this means relatively high production costs. If conversely, slide valves are used in such systems, this offers the advantage of more favorable production costs and reduced vulnerability to tolerances. However, to assure adequate high-pressure tightness, a large overlap of the slide control edges must be assured, which in turn necessitates a long valve stroke of several millimeters on the part of the slide valve. This in turn means that an exact, fast closing motion of a valve piston can be achieved in such an embodiment only with difficulty, since the strong spring forces required to bring about an exact, fast closing motion are not feasible within the installation space inside the injector. In a valve piston embodied as a slide valve, its long stroke requires a large installation space if strong spring forces are to be implemented.